The embodiments of the present invention relate generally to measurement while drilling data transmission technologies. More specifically, the embodiments relate to methods and apparatus for generating pressure pulse signals in a circulating drilling fluid. Still more specifically, the embodiments relate to methods and apparatus for using an electroactive fluid to create pressure pulse signals.
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earthen formations traversed by the wellbore, data relating to the size and configuration of the wellbore itself, and information as to tool orientation, location, and operating parameters. Techniques used to measure conditions in the wellbore, including the movement and location of the drilling assembly, during drilling operations are commonly known as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
These techniques often involve the use of a telemetry system that employs one or more sensors or transducers at the lower end of the drill string that collect data from the drill string or wellbore. These sensors relay the gathered information to an encoder that coverts the data to digital signals, which can be transmitted to receiving equipment at the surface. A commonly employed technique to relay signals from downhole to the surface is transmission of pressure pulses through the column of drilling mud that fills the borehole. These pulses are then received and decoded by a pressure transducer and computer at the surface.
In typical prior art mud pressure pulse systems, the pressure pulses in the drilling mud are created by means of a valve and control mechanism, generally termed a pulser or mud pulser. Mud pressure pulses are generated by opening and closing a valve, normally near the bottom of the drill string, so as to momentarily restrict or increase the mud flow. Early MWD tools used a “negative” pressure pulse that was created in the fluid by temporarily opening a valve in the drill collar allowing direct communication between the high pressure fluid inside the drill string and the fluid at lower pressure returning to the surface via the wellbore annulus. Negative pressure pulse techniques proved less than ideal because a failure in the valve could result in an uncontrolled release of drill string fluid into the annulus.
Alternatively, and often more preferably, a “positive” pressure pulse was created by temporarily restricting the flow of drilling fluid by partially blocking the fluid path in the drill string. Devices used to create these positive pressure pulses include poppets, sirens, and rotary pulsers.
Poppet-type pulsers operate like unidirectional check valves by permitting the flow of fluid in only one direction. The poppet employs an axially moveable plug to open and close a fluid pathway that, when closed, causes a pressure rise in the drilling fluid.
Sirens typically feature a stationary stator and a coaxially mounted, motor driven rotor. The stator and the rotor have a plurality of radially extending lobes such that when the lobes of the stator and the rotor are aligned, a fluid port is formed for the passage of fluid. As the rotor rotates, the flow of fluid is interrupted and pressure pulses are generated.
A rotary pulser is similar to a siren but rather than being driven to produce a relatively continuous series of signals like a siren, the actuation of a rotary pulser is controlled to produce a desired sequence of pulses in the drilling fluid. Thus, instead of the constant rotation of a siren, a rotary pulser is intermittently rotated a small amount to open and close fluid pathways.
Because all of these pulser designs operate by restricting the flow of drilling fluid through relatively small passageways, erosion and wear caused by the abrasive-laden drilling fluid is a serious concern. Drilling fluid normally contains some concentration of solid particles, which, at the pressure and flow rates typically encountered, tend erode the pulser components. Such erosion can lead to relatively short useful lives for many pulser components. Thus, there remains a need in the art for a pulser design exhibiting improved wear characteristics.
Disclosed in U.S. Pat. No. 2,661,596, the entire disclosure of which is hereby incorporated by reference, are electroactive fluids whose viscosity, or resistance to flow, is modifiable by subjecting the fluid to a magnetic or electric field. Electroactive fluids that are responsive to an electrical field are known as electrorheological (ER) fluids, while those responsive to magnetic fields are known as magnetorheological (MR) fluids. Of these two, MR fluids have proved easier to work with because they are less susceptible to performance-degrading contamination, and are easily controllable using magnetic fields easily created with either permanent magnets or electromagnets.
MR fluids can be formed by combining a low viscosity fluid, such as a type of oil, with magnetizable particles to form a viscous slurry. U.S. Pat. No. 2,661,596 used particles of iron on the order of 0.1 to 5 microns, with the particles comprising 20% or more by volume of the MR fluid. More recent work in MR fluids can be found, for instance, in U.S. Pat. No. 6,280,658, the entire disclosure of which is hereby incorporated herein by reference.
When a magnetic field passes through an MR fluid, the magnetizable particles align with the field, limiting movement of the fluid due to the arrangement of the magnetizable particles. As the field increases, the MR fluid becomes increasingly solid, but when the field is removed, the fluid reassumes its liquid state again. MR fluids have been used in such areas as dampers, locks, brakes, and abrasive finishing and polishing. MR fluids can be commercially obtained from the Lord Corporation of Cary, N.C.
The embodiments of the present invention are directed to methods and apparatus for generating a pressure pulse in drilling fluid using a pulser, controlled by an electroactive fluid, that seeks to overcome the limitations of the prior art.